Atlantis MU is a cross platform app but will be available first for android, desktop and mac. It will be integrated with your accounts in Agartha, so no need for another registration, and you will be able to use your existing eCoins to Atlantis.

Atlantis will initially includes the first 3 class of MU (Elf, Wizard and Dark Knight) but with a bunch of new contents (New Castle Siege, Pet system, New Mounts (Flying dragin with character in it), Forging, Boss event etc...) not released in regular MU for you to explore.